1. Field of the Invention
The invention concerns an x-ray system, in particular a computed tomography system, with an x-ray source, a projection detector arrangement associated with the x-ray source for the acquisition of projection data of an examination subject, and a monitor detector to measure current dose measurement data of the x-ray radiation of the x-ray source, wherein the monitor detector is designed and arranged so that it detects a portion of the x-ray radiation that is not traveling through the examination subject. Moreover, the invention concerns a method to acquire projection data of an examination subject by means of such an x-ray system, as well as a method to generate image data.
2. Description of the Prior Art
Computed tomography is based on x-ray radiation that is emitted by an electron beam that strikes a target when a defined voltage is applied between the electron source and the target. The energy, or more precisely the energy spectrum, of the emitted x-ray radiation is dependent on the applied tube voltage, but the x-ray intensity depends on the applied amperage. The emission of x-ray radiation follows the buildup of a voltage and an amperage at the tube with a certain delay. The applied voltage or amperage and the bremsstrahlung that is thereupon emitted normally do not have a rise and fall response that are proportional to one another. Therefore, discrepancies can occur between the x-ray radiation energy that is determined on the basis of the applied voltage, and the actually emitted x-ray radiation energy, and between the x-ray intensity that is calculated on the basis of the applied current, and the actual intensity of the emitted x-rays. These discrepancies can be subject to additional variations, for example over the service life of the tube and due to aging processes.
Monitor detectors that measure an intensity of x-ray radiation emitted in proximity to the x-ray tube are known in computed tomography for the measurement of intensity fluctuations in the radiation emission. In coordination with the main detector of the CT scanner (with which the projection data are acquired), such monitors allow monitoring of the x-ray intensity and provide data for a correction of measurement data of the main detector. Conventional monitor detectors are designed as energy-integrating detectors that total the charges of the occurring x-ray photons in a defined measurement time period. However, this measurement method has limitations that occur (for example given a manner of operation known as dual energy computed tomography) when the x-ray tube is controlled with rapidly changing voltage and/or current strengths for the purposes of improved imaging, and therefore the tube generates x-ray radiation with different energy spectra. In particular in these methods with a fast voltage switching, due to the time delay of the voltage buildup at the x-ray source, particularly severe discrepancies can occur between the x-ray energy determined at the applied voltage and the actual current x-ray energy, and as a result adulterations can occur in the imaging or evaluations based on the image.